For several years a variety of products packaged in bottles or jars having screw-on lids or caps have been provided with an additional inner container sealing material to protect the container contents if the cap or lid is removed. This inner container sealing material is approximately the same diameter as the outer diameter of the container mouth and is adhered to the lip of the container by means of an adhesive. Furthermore, the inner container sealing material usually cannot be removed from the container without evidence of tampering. Additionally, the inner container sealing material prevents air, moisture, and/or other contaminants from contacting the contents of the unopened container and helps prevent leakage of liquids packaged in the unopened containers.
The commercial laminated cap liners that provide an inner container seal include a layer of pulpboard secured to an aluminum foil container sealing material with a microcrystalline wax. The exposed surface of the aluminum foil is provided with a heat-sealable adhesive coating that is tack-free at room temperature. In the usual commercial process, the unexposed surface of the pulpboard backing is permanently glued to the inner upper surface of a threaded cap or lid, that ultimately is mounted on the complimentarily threaded mouth of a filled container. The cap or lid then is passed through an induction heating unit, serving to rapidly heat the aluminum foil and simultaneously melt the microcrystalline wax and the heat-sealable adhesive coating. The heat-sealable adhesive coating then acts to adhere the container sealing material to the container. The melted microcrystalline wax is quickly absorbed by the porous pulpboard or paper backing, therefore greatly reducing the strength of bond between the pulpboard backing and the aluminum foil, such that when the cap or lid is removed from the container, the pulpboard backing remains secured to the inside of the cap or lid, while the aluminum foil remains firmly sealed to the mouth of the container. This well-known prior art cap liner is widely used because of its simplicity of application, and further details of this induction heating cap sealing process are set forth in considerable detail in Palmer U.S. Pat. No. 2,937,481.
As described above, prior to the present invention, the typical structure of a cap liner designed for use with induction heating cap sealing techniques includes a layer of pulpboard secured to a layer of aluminum foil by a layer of microcrystalline wax. The opposite face of the aluminum foil is provided with a heat-sealable adhesive coating that is compatible with the material of construction of the container. Caps or lids provided with such a laminated cap liner are torqued onto filled containers, and these containers then are passed through a high frequency induction heating unit. During the induction heating process, the aluminum foil usually reaches a temperature in the range of about 200.degree. F. to about 325.degree. F. causing the microcrystalline wax to melt and be absorbed by the pulpboard, and causing the container sealing material to be heat welded to the rim of the container by the heat-sealable adhesive coating.
Although resealable caps including the above-described liners are easy to apply to the container, the manufacturing process of the above-described prior art laminated cap liners is a complex process requiring sophisticated and expensive equipment, such as heating tanks, special pumps and special applicators in order to apply the microcrystalline wax adhesives to laminate the pulpboard backing, and a chilled rolling system in order to set the laminate. In addition to being cumbersome and expensive, this manufacturing process requires high energy consumptions and presents an operator hazard in handling melted microcrystalline waxes at temperatures of about 160.degree. F. to about 180.degree. F. Furthermore, the laminated cap liner resulting from this manufacturing process does not allow the cap and/or container to continue protecting the container contents from the effects of air and moisture and from leakage after the container sealing material is removed from the container by the consumer.
However, in accordance with the present invention, a water-based settable and/or crosslinkable adhesive composition is used to replace the microcrystalline wax in the production of laminated cap liners. As will be discussed more fully hereinafter, the water-based settable and/or crosslinkable adhesive composition affords the benefit of a simple and economic application to the pulpboard and the benefit of forming a leak-resistant polymeric film over the pulpboard backing upon crosslinking of a crosslinkable adhesive composition and/or upon setting of a non-crosslinking adhesive composition during the induction heating cap sealing process to release the container sealing material. In contrast, the microcrystalline wax utilized in the prior art laminated cap liners merely melts during the induction heating process and is absorbed by the relatively absorbent pulpboard backing to release the container sealing material. The absorptivity of the pulpboard is not effectively reduced by the absorbed microcrystalline wax. Therefore, after the container is opened and the protective inner container seal is removed, the prior art laminated cap liners are subject to leakage because the container contents can contact and can be absorbed by the pulpboard backing, thereby eventually deteriorating the pulpboard backing and leading to container leakage at the cap area. However, the water-based settable and/or crosslinkable adhesive compositions utilized in the present invention serve initially to secure the container sealing material to the pulpboard backing while the settable and/or crosslinkable adhesive composition is still tacky and essentially unset and/or uncrosslinked. Then, after setting and/or crosslinking initiated by the induction heating process, the water-based adhesive composition forms a chemically-resistant, liquid impermeable barrier coating over the pulpboard to prevent direct contact of the pulpboard with the contents of the container. By preventing direct contact between the container contents and the pulpboard, pulpboard disintegration and the resulting leakage of the contents at the cap area of the container is precluded.
As described above, the laminated cap liner structure of the present invention is an improved laminated cap liner wherein the microcrystalline wax used in the prior art to secure the container sealing material to the pulpboard backing is replaced by a crosslinkable adhesive composition. Other investigators have thoroughly studied the heat-sealable adhesive coatings that are applied to the face of the container sealing material that contacts the mouth of the container and act to bond the container sealing material to the mouth of the container. For example, Ou-Yang, in U.S. Pat. No. 4,503,123, discloses a blend of an acrylate polymer and a vinyl polymer to act as an adhesive to allow an aluminum foil container sealing material to bond to either glass or plastic containers. However, Ou-Yang, in accordance with the prior art, uses a microcrystalline wax to secure the aluminum foil container sealing material to the pulpboard backing. Furthermore, in direct contrast to the crosslinkable adhesive composition used in the present invention to secure the pulpboard or paper backing to the face the container sealing material opposite the face contacting the mouth of the container, the adhesives disclosed by Ou-Yang are tack free at room temperature, and upon induction heating, the resulting increase in temperature increases the tack of the heat-sealable adhesive to permit bonding of the aluminum foil container sealing material to the container. In accordance with the present invention and in contrast to the prior art, the settable and/or crosslinkable adhesive composition utilized to secure the pulpboard or paper backing to the container sealing material possesses sufficient tack before undergoing the induction heating process such that the aluminum foil container sealing material can releasably bond to the pulpboard backing, and, upon induction heating and the resulting increase in temperature, the settable and/or crosslinkable adhesive composition loses its tack to release the aluminum foil container sealing material and to form leak-resistant, liquid impermeable settable and/or crosslinked polymeric film over the pulpboard backing.
In addition, Hori et al in U.S. Pat. No. 4,199,646, discloses heat activated adhesives that increase in tack as the temperature is increased. When utilized as a pressure-sensitive adhesive, the adhesives disclosed by Hori substantially increase in tack as temperatures are elevated from ambient temperature to 60.degree. C. to 100.degree. C. The Hori et al patent is directed to adhesives that are substantially non-viscous at normal temperatures and have excellent bond strength at high temperatures. The settable and/or crosslinkable adhesive compositions utilized in the present invention similarly are non-viscous at normal temperature. However, in accordance with the present invention, the settable and/or crosslinkable adhesive compositions lose their bond strength (tackiness) at higher temperatures to form non-adhesive, chemically-resistant films.
The water-based settable and/or crosslinkable adhesive compositions used in the present invention exhibit a sufficient bond and tack at room, or ambient, temperature to bond the pulpboard or paper backing to the container sealing material. However, upon induction heating, wherein the temperature of the container sealing material can rise to from about 100.degree. F. to about 425.degree. F., the water-based settable and/or adhesive compositions set and/or crosslink to essentially lose all of their tack and produce a continuous polymeric film such that the container sealing material can be released from the pulpboard or paper backing for bonding to the container by a heat-sealable adhesive coating, such as those disclosed by Ou-Yang in U.S. Pat. No. 4,503,123. In contrast, induction heating of the container sealing material would not serve to set and/or crosslink the adhesives disclosed by Hori to form a non-adhesive film and therefore release the container sealing material. Conversely, the adhesives disclosed by Hori would maintain, or increase, their strong adhesive characteristics at the elevated temperatures achieved by the induction heating cap sealing process and not release the container sealing material. The net result would be a container sealing material bound both to the container and to the pulpboard backing, therefore making initial cap removal from the container difficult or impossible, reducing tamperproof effectiveness, and destroying any leakproof characteristics of the cap liner upon resealing of the container.